molecules Article Is It Conjugated or Not? The Theoretical and Experimental Electron Density Map of Bonding in p-CH3CH2COC6H4-C≡C-C≡C-p-C6H4COCH3CH2 Przemysław Starynowicz , Sławomir Berski, Nurbey Gulia , Karolina Osowska, Tadeusz Lis and Sławomir Szafert * Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland; [email protected] (P.S.); [email protected] (S.B.); [email protected] (N.G.); [email protected] (K.O.); [email protected] (T.L.) * Correspondence: [email protected] Academic Editors: Roman Dembinski and Maxim L. Kuznetsov Received: 15 July 2020; Accepted: 19 September 2020; Published: 24 September 2020 Abstract: The electron density of p-CH CH COC H -C CC C-p-C H COCH CH has been 3 2 6 4 ≡ ≡ 6 4 3 2 investigated on the basis of single-crystal X-ray diffraction data collected to high resolution at 100 K and from theoretical calculations. An analysis of the X-ray data of the diyne showed interesting “liquidity” of electron distribution along the carbon chain compared to 1,2-diphenylacetylene. These findings are compatible with the results of topological analysis of Electron Localization Function (ELF), which has also revealed a larger (than expected) concentration of the electron density at the single bonds. Both methods indicate a clear π-type or “banana” character of a single bond and a significant distortion from the typical conjugated structure of the bonding in the diyne with a small contribution of cumulenic structures. Keywords: polyynes; density map; single-crystal X-ray diffraction; theoretical calculations; electron localization function 1. Introduction Within the last three decades, research on organic, organometallic and metal-containing polyynes has focused a lot of attention and witnessed a series of stunning breakthroughs [1,2]. Such compounds of rigid-rod architecture are envisioned as precursors of still elusive sp-hybridized carbon allotrope—carbyne. Yet another important factor that significantly spurred the research in this area is the extraordinary (and somehow surprising) physical properties of such “material” that have been predicted by theorists. They suggest (based on theoretical calculations) that carbyne possesses for instance, a tensile strength that surpasses that of carbon nanotubes, graphene, and diamond [3–5]. In order to better understand the physical and chemical properties of polyynes and gain a deeper insight into the detailed electronic structure of a carbon chain, various series of oligo and polyynes of type X(C C)nX have been synthesized and thoroughly studied [6–26]. The received data allowed to ≡ define the effect of a chain length upon various molecular properties and quantify a dependence of many measurable physical values, like an absorption band, redox potential, or NMR chemical shifts against 1/n (where n is the number of alkyne units). The structure of the polyyne chain itself is of great interest from several standpoints. One of the most intriguing questions is: With the chain lengthening, will the triple and single bond equalize (a vanishing HOMO/LUMO energy gap) or approach two different values (a persistent energy gap; from a solid-state physics perspective, a Peierls distortion) [27,28]? Another good question is: To what Molecules 2020, 25, 4388 ; doi:10.3390/molecules25194388 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 11 vanishing HOMO/LUMO energy gap) or approach two different values (a persistent energy gap; Moleculesfrom a solid2020, 25-state, 4388 physics perspective, a Peierls distortion) [27,28]? Another good question is:2 of T 11o what degree carbyne can easily bend? There has been conjecture that distortion of long sp carbon chains might trigger isomerization to fullerenes or other sp2 carbon allotropes [29–35]. degreeIn carbynethe 2003 can review easily bend?article There(followed has beenby the conjecture 2006 up thatdate distortion paper), ofall long crystallographically sp carbon chains 2 mightcharacterized trigger isomerizationcompounds with to fullerenes at least eight or other consecutive sp carbon sp allotropes hybridized [29 –carbons35]. (four conjugated C≡C Inbonds) the 2003 were review analyzed article (followed[36]. The bymajor the 2006interest update—at paper), the molecular all crystallographically level—were bond characterized lengths, compoundsbond angles, with and at sp least carbon eight consecutivechain conformations. sp hybridized Average carbons values (four conjugatedderived from C C bond bonds) length were ≡ analyzedsuggested [ 36that]. The(1) as major the midpoints interest—at of thethe molecularsp carbon level—werechains are approach bond lengths,ed, the bondC-C bonds angles, contract and sp carbonand the chainC≡C bonds conformations. lengthen, and Average (2) as values the chains derived are extended from bond to lengththe macromolecular suggested that limit (1) asof the midpointsone-dimensional of the sp car carbonbon allotrope chains are (carbyne), approached, the C the-C C-C bonds bonds contract contract to andthe proposed the C C bonds limiting lengthen, value ≡ and1.32– (2)1.33 as Å the and chains the C≡C are extended bonds lengthen to the macromolecular to 1.25 Å (the effect limit ofof thethe one-dimensional error limits on the carbon bond allotrope lengths (carbyne),(esd values) the is omitted). C-C bonds contract to the proposed limiting value 1.32–1.33 Å and the C C bonds ≡ lengthenAlthough to 1.25 in Å further (the eff ectapproximations of the error limits of these on the limits bond, computational lengths (esd values) chemists is omitted). will still play a crucialAlthough role we inbelieve further some approximations additional important of these limits, experimental computational values chemists can be gather will stilled. play This a crucialwould roleof course we believe, include some precise additional crystallographic importantexperimental electron density values maps can of be bonding gathered. in Thispolyynes. would Although, of course, includewhat is preciseobvious crystallographic, the main interest electron should density be directed maps ofat bonding longer poly in polyynes.ynes as an Although, initial point what we is obvious,present here the mainthe map interest of a shoulddiyne with be directed a carbonyl at longer group polyynes containing as an phenyl initial endgroups, point we present which here is next the mapcompared of a diyne with witha theoretical a carbonyl one. group containing phenyl endgroups, which is next compared with a theoretical one. 2. Results and Discussion 2. Results and Discussion The experimental difficulties of the research on electron density maps are worth mentioning. First,The the experimentalhighest quality diffi crystalsculties ofare the required research. Second, on electron the compound density maps and are its worth crystals mentioning. must be First,extremely the highest stable qualityand should crystals not are change required. (cracking, Second, thephase compound transitions, and itsetc.) crystals during must an beexperiment. extremely stableThird, andthere should must not not change be any (cracking, disorder phase in the transitions, measured etc.) crystal during and an experiment.data must be Third, collected there mustand notprocessed be any disorderwith the inutmost the measured care. Long crystal-lasting and dataoptimization must be collected of the crystallization and processed process with the (speed, utmost care.temperature, Long-lasting concentration) optimization and pure of the luck crystallization allowed us processto finally (speed, obtain temperature,crystals of sufficiently concentration) good andquality pure to luck constitute allowed a usproper to finally data obtainsource crystals for such of an su ffiexperiment.ciently good The quality crystal to constitutestructure was a proper first datasolved source conventionally for such an and experiment. the results Thehave crystal already structure been published was first [3 solved7]. conventionally and the results have already been published [37]. The diyne p-CH3CH2COC6H4-C≡CC≡C-p-C6H4COCH3CH2 (1) was synthesized as described earlierThe and diyne it pwas-CH fully3CH2 COCcharacterized6H4-C CC includingC-p-C6H 14HCOCH- and3 CH13C-2NMR(1) was and synthesized IR spectroscopy, as described MS ≡ ≡ 1 13 earlierspectrometry and it was and fully X-ray characterized crystallography including [37].H- Next and, dataC-NMR for andthe IRcarefully spectroscopy, selected MS spectrometrycrystal were andcollected X-ray as crystallography described in the [37 Experimental]. Next, data forSection. the carefully The view selected of the crystalstructure were is collectedpresented as in described Figure 1 inand the the Experimental metrical parameters Section. from The conventional view of the structure(CR) and ismultipole presented (MP) in Figurerefinement1 and are the compared metrical parametersin Table 1. from conventional (CR) and multipole (MP) refinement are compared in Table1. Figure 1. View of 1. Table 1. Bond lengths in 1. Table 1. Bond lengths in 1. CR CRMP MP CRCR MP MP O(1)-C(3)O(1)-C(3) 1.2231(8)1.2231(8) 1.2248(4) 1.2248(4) C(11)-C(12)C(11)-C(12) 1.4077(8)1.4077(8) 1.4080(5) 1.4080(5) C(1)-C(2)C(1)-C(2) 1.2120(8)1.2120(8) 1.2169(4) 1.2169(4) C(11)-C(16)C(11)-C(16) 1.4055(8)1.4055(8) 1.4060(5) 1.4060(5) C(1)-C(11)C(1)-C(11) 1.4300(9) 1.4300(9) 1.4259(4) 1.4259(4) C(12)-C(13)C(12)-C(13) 1.3933(9)1.3933(9) 1.3934(4) 1.3934(4) C(2)-C(2A)C(2)-C(2A) 1.3670(11) 1.3670(11) 1.3636(6) 1.3636(6) C(13)-C(14)C(13)-C(14) 1.3981(8)1.3981(8) 1.3987(4) 1.3987(4) C(3)-C(4) 1.5141(8) 1.5109(4) C(14)-C(15) 1.4039(8) 1.4039(4) C(3)-C(4) 1.5141(8) 1.5109(4) C(14)-C(15) 1.4039(8) 1.4039(4) C(3)-C(14) 1.5016(9) 1.5008(4) C(15)-C(16) 1.3908(9) 1.3924(4) C(3)-C(14) 1.5016(9) 1.5008(4) C(15)-C(16) 1.3908(9) 1.3924(4) C(4)-C(5) 1.5202(9) 1.5237(5) C(4)-C(5) 1.5202(9) 1.5237(5) Molecules 2020, 25, 4388 3 of 11 As it can easily be noticed, the inclusion of the multipole parameters has led to just subtle modifications of the bond lengths.